1. Field of the Invention
The invention relates to controlling reactor efficiency of a pressurized water nuclear reactor by boric acid content in the coolant.
2. Brief Description of Related Art
It is known that boric acid in the cooling medium of a nuclear reactor, used for control of the fuel reactivity, causes an increased material corrosion. Moreover, during the decomposition of the nuclear fuel strong flows of neutrons, gamma-radiation and .alpha. or .beta. particles act on the water and decompose the same into oxygen, hydrogen and a plurality of oxide radicals which increase material corrosion (Report of IAEA "Coolant Technology of Water Reactors", Doc. 0846j, Mar. 11, 1981 S. 27-29).
The corrosion products get into the radiation core in the form of insoluble oxides through neutron flow (fuel elements). They become radioactive, settle on the inner face of the reactor during feeding through the coolant cycle and form a radioactive contamination causing considerable technical difficulties during the operating, repairing and inspection of the reactor (Report of IAEA "Coolant Technology of Water Reactors", Doc. 0846j, Mar. 11, 1991, S. 54-55).
For reducing the corrosivity of the boric acid in the pressurized water reactors, 1 to 2 ppm LiOH (Report of IAEA "Coolant Technology of Water Reactors", Doc. 0846j, Mar. 11, 1991, S. 27-29) are added. Alternatively, taking into consideration the presence of ions like Li.sup.+ and NA.sup.+, 0.05-0.45 mM/kg KOH may be added.
For reducing the radiolysis of the water, hydrogen is added in pressurized water reactors and in water-water-reactors. Ammonia from which hydrogen is then released (Report of IAEA "Coolant Technology of Water Reactors", Doc. 0846j, Mar. 11, 1991, S. 27) may be added.
These methods permit the operation of nuclear reactors, however, do not assure any considerable lowering of the contamination of the reactor, i.e., the gamma-dosage efficiency of the individual parts of the steam generator is in a range of between 3 to 30 R/h (see Report of IAEA "Coolant Technology of Water Reactors", Doc. 08461, Mar. 11, 1991, S. 27-28).
The nuclear reactor operates in a known method in accordance with FIG. 1 as follows:
The primary cycle includes a reactor 1, a main circulating pump 2, ion exchange filter 3 for cleaning the coolant, a steam generator 4, a steamline 5 of the secondary cycle, a pump 6 for discharging the coolant, a feeding ventilator 7, cycle pipe lines 8 having a large diameter, a feeding pump 9, a pressure holder 10, a pressure reduction device 11 for ventilator and fuel elements 12 and a coolant A.
A dosage of 12 to 15 g/l boric acid solution is added to the primary cycle of reactor 1 which is fed with fresh fuel material. The ion exchange filter 3 for cleaning the coolant A is saturated with H.sub.3 BO.sub.3 and KOH in a mixture with NH.sub.3 up to a balanced concentration. Before operating the reactor 1 at a temperature of no more than 80.degree. C. a hydrazine hydrate solution is added to the coolant A in a concentration which is above a three fold excess of the measured O.sub.2 content for a chemical binding of the oxygen.
After executing the above mentioned operating steps the reactor is then physically accelerated until reaching a controlled minimum efficiency (W-10.sup.-6 %).
After the required measuring of the physical parameter of the reactor plant, the reactor efficiency is then further accelerated up to the level of energy generation. At this point KOH is added to the coolant in an amount corresponding to the measured H.sub.3 BO.sub.3 amount, however, no more than 0.45 mMol/kg and 5 to 10 mg/kg NH.sub.3, with respect to the coolant. Thereafter, H.sub.2 is released from NH.sub.3 in view of the radioalysis 30 to 60 n.ml/kg, with respect to the coolant. With these chemical parameters the commercial operation of water-water-reactors is started. Since the radiolysis of the water occurs with a high speed due to the radiation effect and in that the reunification of the oxide radicals does not occur immediately during its reaction with H.sub.2, the oxide radicals of the water result in material corrosion, whereby insoluble Co, Mn, Ni, Cr, Fe and other elements containing oxide compounds are generated.
Due to the coolant cycle the corrosion products are fed to the effective range of the nuclear fuel and are attracted by the wall of the fuel elements due to the released thermoelectric forces, whereby they are subjected to a tremendous neutron flow and convert into isotopes .sup.58 Co, .sup.60 CO, .sup.54 Mn, .sup.51 Cr, .sup.59 Fe etc.
Under a constant reactor load the activated corrosion products slowly change over into the coolant water. However, during severe load fluctuations these products are violently washed off the walls of the fuel elements. However, in both cases metal oxide particles are distributed through the coolant cycle and deposit on the reactor walls due to isotope exchange or merely by sorption on the inner faces of the reactor (sorption coefficient K.sup.+ 1.multidot.10.sup.-4 sec. .sup.-1) and therefore result in contamination. Equipment for cleaning the cooling medium reduces the content of activated products in the coolant with a coefficient K.sup.+ 1.multidot.10.sup.-5 sec. .sup.-1 by removing a certain portion of the coolant. However, a complete cleaning of the coolant is not obtained with this measure. Finally a slow enrichment of radioactive isotopes builds up on the inner reactor face and results in contamination of the same. The known techniques enable the operation of nuclear reactors, however do not assure a considerable lowering of the contamination of the reactor, i.e., the Y-dosaging efficiency of the individual parts of the steam generator is in a range of between 3 and 30 R/H. The lowest Y-radiation dosage is obtained by adding KOH and NH.sub.3 into the primary cycle of water-water-reactors. However, in this case the gamma-radiation dosage is still very high (at an average of 3 to 7 R/h), as is the case in the power plant in Lovisa (Finland), for example. The gamma-dosage efficiency ranges in pressurized water reactors of the type "Westinghouse" and "Siemens" (KWU) at 3 to 40 R/h or 3 to 45 R/h (see report of IAEA "Coolant Technology of Water Reactors", Doc. 0845j, Nov. 03, 1991, S 54-55).
Hence, nuclear plants which are presently in operation show the following disadvantages with respect to the water chemical operation:
1. Insufficient speed of the reunification of the oxide radicals under H.sub.2 effect, whereby a corrosion of the reactor material occurs.
2. the combustion element wall, which results in its activation and subsequent migration through the coolant cycle.
3. The sorption coefficient of the activated corrosion products exceeds the cleaning coefficient with respect to the settling speed, so that in spite of the efficiency increase of the cleaning means a contamination of the reactor cannot be prevented.
It is an object of the invention to decrease the speed of material corrosion as well as to reduce the activation of the corrosion products and its depositing on the reactor walls, and in this manner to reduce the reactor contamination by a corresponding reduction of the gamma dosage efficiency to &lt;3 R/h.